The present invention relates to field effect transistors and more specifically to biasing circuits for use in conjunction with mixers which utilize field effect transistors.
Mixers which use field effect transistors in radio frequency applications are subject to conversion loss problems which are a function of temperature and process conditions. As shown in FIG. 4 by curves 5 and 7 representing different local oscillator power levels, changes in the gate bias voltage which commonly occur as a result of temperature and process variations have a substantial effect on the amount of conversion loss in the mixer and seriously limit mixer performance. Minimum conversion loss is achieved when the field effect transistors are biased at or near their threshold or pinch-off voltages. However, since the threshhold voltage which is characteristic of a particular field effect transistor changes as a function of temperature and process conditions it is desirable to provide some form of dynamic biasing which responds to such changes in order to maintain bias levels in proximity to threshhold voltages.
It is therefore an object of the present invention to provide a biasing circuit for use with field effect transistors which dynamically biases the transistors in response to process and temperature changes.
It is a further object of the present invention to provide a field effect transistor ring mixer having a biasing circuit which responds to process and temperature changes to maintain the bias level at or near the threshhold voltages for the transistors in the mixer.
It is further object of the present invention to provide a biasing circuit for a radio frequency mixer employing field effect transistors which provides minimum conversion loss over a range of temperature and process conditions.